(1) Field of the Invention
This invention relates to a method for the prevention of post oxidation of methacrolein.
(2) Description of the Prior Art
Methacrylic acid is generally produced by a process, which comprises a former-stage reaction and a latter-stage reaction. In the former-stage reaction, isobutylene, tertiary butanol, methallyl alcohol or the like is used as a starting material. In the presence of an oxidation catalyst of the molybdenum-bismuth-iron system, the starting material is subjected at 300.degree.-450.degree. C. to vapor-phase oxidation with a molecular-oxygen-containing gas so that methacrolein is obtained primarily. In the latter-stage reaction, in the presence of a multi-element molybdate catalyst, the methacrolein obtained by the former-stage reaction is subjected at 250.degree. -400.degree. C. to vapor-phase oxidation with a molecular-oxygen-containing gas a in the former-stage reaction, whereby methacrylic acid is obtained.
In the former-stage reaction, a reaction product gas flowed out of a reaction tube at a high temperature of at least 300.degree. C. as mentioned above is abruptly reduced in linear velocity in an empty column portion provided at an outlet of the reaction tube, so that oxidation of methacrolein with unreacted oxygen, which may be regarded as a post reaction, takes place to form carbon monoxide, carbon dioxide and the like.
The oxidation (hereinafter referred to as "post oxidation") of methacrolein with the unreacted oxygen leads to a decrease in the yield of methacrolein, whereby the yield of methacrylic acid from isobutylene, tertiary butanol or methallyl alcohol is lowered. It is hence necessary to prevent the post oxidation. For the prevention of the post reaction, it has been known effective to lower the temperature of a product gas immediately after the product gas has flowed out of the reaction tube. It has therefore been known to provide a cooler immediately after an outlet of a reaction tube or to spray water to an outlet portion of a reaction tube so as to cool the same (Japanese Patent Laid-Open No. 54317/1974).
When the cooler is provided at a location immediately downstream of the outlet of the reaction tube, the cooler is however assembled as a unitary element with the reactor so that the production facilities become complex and large and replacement of a catalyst or the like is rendered complex. In the spraying of water to the outlet portion of the reaction tube, the reaction product gas is prone to excessive over-cooling partially or locally so that high boiling substances, for example, terephthalic acid and trimellitic acid contained in the reaction product gas are caused to deposit, thereby causing their sticking on line walls or blocking of lines.